Thin film photovoltaic device and process of manufacture

ABSTRACT

Provided is a thin film photovoltaic device and a method of manufacturing the device. The thin film photovoltaic device comprises a film layer having particles which are smaller than about 30 microns in size held in an electrically insulating matrix material to reduce the potential for electrical shorting through the film layer. The film layer may be provided by depositing preformed particles onto a surrogate substrate and binding the particles in a film-forming matrix material to form a flexible sheet with the film layer. The flexible sheet may be separated from the surrogate substrate and cut into flexible strips. A plurality of the flexible strips may be located adjacent to and supported by a common supporting substrate to form a photovoltaic module having a plurality of electrically interconnected photovoltaic cells.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under Subcontract No.ZN-0-19019 awarded by the Midwest Research Institute, National RenewableEnergy laboratory Division under Prime Contract No. DE-AC02-83CH10093awarded by the Department of Energy. The Government has certain rightsin this invention.

FIELD OF THE INVENTION

The present invention relates generally to photovoltaic devices andmethods of manufacturing the same.

BACKGROUND OF THE INVENTION

Photovoltaic cells have long been considered a possible alternative tononrenewable energy sources. Most types of photovoltaic cells have aphotoactive area (i.e., an area that generates electricity in responseto light) that is composed of a semiconductor layer disposed between twoelectrode layers. The various layers are typically deposited on a flat,rigid supporting substrate, such as glass. A number of electricallyinterconnected photovoltaic cells may be supported on a commonsupporting substrate. The combination of the supporting substrate andthe photovoltaic cell(s) is typically referred to as a photovoltaicmodule.

One process for making a photovoltaic module is to sequentially formlayers on top of a supporting substrate to build a photoactive area onthe supporting substrate. The photoactive area may then be divided intoa number of individual photovoltaic cells which may be interconnected inseries to boost the voltage output from the photovoltaic module. Thephotovoltaic cells may then be encapsulated to protect the cells fromthe external environment during operation of the module. The process ofbuilding a photovoltaic module by the sequential formation of layers ona supporting substrate is a common technique for making thin filmphotovoltaic devices, such as those using thin semiconductor films ofcadmium telluride or copper indium diselenide in a power-producingsemiconductor layer.

The sequential deposition of layers on a supporting substrate such asglass, however, is expensive. Some deposition steps may require a hightemperature, which requires energy to heat the supporting substrate andinduces thermal stresses into the supporting substrate which may causebreakage of some substrate materials, such as glass. Also, theprocessing of glass sheets is cumbersome and not well suited to a highspeed operation.

Furthermore, for some thin film semiconductor materials, such as cadmiumtelluride, the semiconductor material is typically recrystallizedfollowing deposition of the semiconductor material to obtain crystals ofa suitable size and morphology for use in a photovoltaic cell.Recrystallization tends to be, however, a very slow process and requiresa significant amount of energy to heat not only the desiredsemiconductor film but also the supporting substrate, significantlyadding to the expense of the manufacturing process. Additionally, duringrecrystallization, void spaces tend to form in the semiconductor film,making the semiconductor film more susceptible to electrical shortingthrough the film.

Another problem encountered with many thin film photovoltaic devices iscontrol over the stoichiometry of the components of the semiconductormaterial during deposition of semiconductor thin films. For example,maintenance of the stoichiometry of copper indium diselenide duringdeposition of a thin film can be difficult to control, especially in ahigh volume industrial setting.

There is a need for an improved process for making photovoltaic devices,and especially for making thin film devices, which is more suitable forhigh volume production and which reduces the costs associated withprocessing the supporting substrate.

SUMMARY OF INVENTION

In one aspect of the present invention small, preformed particles of acrystalline semiconductor material are incorporated into thin filmphotovoltaic cells substantially without the need for recrystallizingthe semiconductor material. In another aspect, the present inventionprovides a method for making photovoltaic devices, and especially thinfilm devices, in which flexible strips having film layers are preparedwhich may be used in forming a photovoltaic module. The flexible stripsare prepared independent from the final, supporting substrate to be usedin the photovoltaic module, thereby reducing the need to process thesupporting substrate during the manufacturing process. The expense ofheating the supporting substrate during the manufacturing process issignificantly reduced.

In one embodiment, the film into which the particles are incorporatedhas a thickness of smaller than about 30 microns, which may be used in athin film photovoltaic device. Preferred semiconductor materials for theparticles are compound semiconductors useful in thin film devices, suchas metal chalcogenides, which have large absorption coefficients andshort diffusion lengths. Particularly preferred as semiconductors foruse in the particles are cadmium telluride and copper indium diselenide.The particles, which are smaller than about 30 microns in size, areincorporated into a thin film of a photovoltaic cell, which could beused in a photovoltaic module, substantially without altering the bulkcrystal morphology of the semiconductor material in the particles.Because the semiconductor material is provided in small, preformedparticles having the desired material properties for use in a thephotovoltaic device, the need, and expense, for controlling thestoichiometry during deposition of a semiconductor material in a thinfilm is reduced. This should be particularly beneficial when usingcopper indium diselenide, the stoichiometry of which is difficult tocontrol during thin film deposition. With the present invention, thesemiconductor particles could be preformed in a more controllableenvironment without the added complexity of simultaneously depositing athin film. Also, the need to recrystallize the semiconductor materialafter deposition in a thin film is reduced because the preformedparticles used to make a thin film according to the present inventionhave a bulk crystal morphology suitable for use in the finalphotovoltaic device. This should be particularly beneficial in makingphotovoltaic devices having cadmium telluride thin films, which havetypically required recrystallization following deposition.

In one embodiment, a flexible sheet, having one or more film layers, isinitially formed on a surrogate substrate. The surrogate substrate maybe flexible and relatively thin. The need is thereby reduced forprocessing a large, bulky substrate, such as a sheet of glass, on whichphotovoltaic cells may ultimately be permanently supported for use.

In another embodiment, preformed particles having a desiredsemiconductor material in a crystalline form may be embedded in aflexible tape which serves as a surrogate substrate. A film-formingmaterial may then be used to fill spaces on the surrogate substratebetween the particles. The film-forming material may be cured to producea thin film having the semiconductor particles held in a matrix of thecured film-forming material. The film-forming material, after curing,should be electrically non-conducting so that the potential forelectrical shorts through the film would be reduced. Additional filmlayers for the photovoltaic device may be deposited on top of thefilm-forming material. For example, when using cadmium tellurideparticles, a heterojunction may be formed on the surrogate substratebetween the cadmium telluride particles and a subsequently depositedsemiconductor film, such as a film of cadmium sulfide. Eventually, theflexible sheet, having the film layer(s), is separated from thesurrogate substrate so that the film layer(s) may be incorporated into aphotovoltaic cell to be supported on a permanent, supporting substrate.The flexible sheet may be cut into pieces to permit a plurality ofphotovoltaic cells to be supported on a common supporting substrate andelectrically interconnected to form a photovoltaic module.

In another aspect, the present invention provides a photovoltaic modulehaving photovoltaic cells having a thin film with particles of asemiconductor material in a crystalline form in which spaces in the filmbetween the particles are occupied by an electrically insulating matrixmaterial, such as an electrically insulating polymeric material. Otherelectrically insulating matrix materials include glass and ceramics. Thephotovoltaic cells thus have a reduced susceptibility to electricalshorting through the film. This provides a significant benefit relativeto current cadmium telluride devices, which are susceptible to shortingthrough the cadmium telluride film due to the presence of void spacesformed in the film during deposition or recrystallization of the cadmiumtelluride. In one embodiment, the matrix material comprises a materialwhich is capable of redirecting energy from sunlight striking the matrixmaterial to the semiconductor particles, thereby reducing the loss topower production due to sunlight striking the matrix material ratherthan a particle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial perspective view of one embodiment of anunencapsulated photovoltaic module of the present invention having aplurality of electrically interconnected photovoltaic cells supported ona common supporting substrate;

FIG. 2 shows a partial cross-section of one embodiment of a photovoltaiccell of the present invention;

FIG. 3 shows a partial cross-section of another embodiment of aphotovoltaic cell of the present invention;

FIG. 4 shows a partial cross-section of yet another embodiment of aphotovoltaic cell of the present invention;

FIGS. 5-7 show a process flow schematic of one embodiment of the presentinvention for making flexible strips which could be used in aphotovoltaic device;

FIGS. 8-10 show in partial cross-section a photovoltaic module invarious stages of manufacture according to one embodiment of the presentinvention;

FIGS. 11-13 show in partial cross-section a photovoltaic module invarious stages of manufacture according to another embodiment of thepresent invention;

DETAILED DESCRIPTION

The present invention provides a photovoltaic device and a method formaking the photovoltaic device that is adaptable to use of a high speed,roll-to-roll operation for forming film layers, such as may be performedon a printing press or similar apparatus. An important aspect of thepresent invention is the construction of film layers for photovoltaiccells on a flexible, surrogate substrate before the layers are locatedon top of a final, supporting substrate. This process simplifies notonly formation of the film layers and the resulting photovoltaic cells,but also division and interconnection of photovoltaic cells on thesupporting substrate. Another important aspect of the present inventionis that small, preformed particles having a semiconductor material incrystalline form may be incorporated into a film layer, substantiallywithout altering the bulk crystal morphology of the semiconductormaterial following deposition of the film.

FIG. 1 shows a partial view of one of the embodiments of a photovoltaicmodule 100 of the present invention which has a plurality ofphotovoltaic cells 102 supported on a supporting substrate 104. Thephotovoltaic cells 102 are electrically interconnected in series acrossinterconnection regions 106 to boost the total voltage of thephotovoltaic module.

The supporting substrate 100 is typically made of a rigid material toprovide structural support to and protection of the photovoltaic cells102 during use. A sheet of glass is a preferred supporting substrate104. Although not shown, the photovoltaic cells, may be encapsulated byany suitable encapsulating structure to protect them for the externalenvironment during operation. One encapsulation structure is disclosedin co-pending U.S. patent application Ser. No. 08/095,381, now U.S. Pat.No. 5,460,660 the contents of which are incorporated herein by referencein their entirety.

With reference to FIG. 2, one embodiment of a photovoltaic cell 102 ofthe present invention is shown. The photovoltaic cell 102 has a firstelectrode film 110, a semiconductor layer 112 and a second electrodefilm 114. The semiconductor layer 112 is capable of generatingelectrical power in response to sunlight and is electricallyinterconnected with both the first electrode film 110 and the secondelectrode film 114. The first electrode film 110 is in electricalcontact with one side of the semiconductor layer 112 and the secondelectrode film 114 is in electrical contact with the other side of thesemiconductor layer 112. During operation, if an external electricalload is connected to the photovoltaic cell 102, electrical current couldflow between the first electrode film 110 and the second electrode film114.

The semiconductor layer shown 112 in FIG. 2 comprises a heterojunction116 between a first semiconductor film 118 and a second semiconductorfilm 120. The second semiconductor film 120 is a thin film, having awidth of smaller than about 30 microns, and includes a plurality ofparticles 122 comprising a second semiconductor material. Spaces betweenthe particles 122 are substantially filled with a matrix material 124which is substantially electrically insulating. The matrix material 124substantially reduces the potential for defects in the secondsemiconductor film 120 which could cause electrical shorting.

The second semiconductor material is in a crystalline form in theparticles 122. Although each particle may comprise multiple crystalgrains, it is preferred that each particle 122 comprise a single crystalgrain. The crystals of second semiconductor materials in the particles122 have a morphology which is suitable for photovoltaic powergeneration. The second semiconductor material is preferably a compoundsemiconductor and is more preferably a metal chalcogenide selected fromthe group consisting of a metal sulfide, metal selenide and metaltelluride, such as are often used in thin film photovoltaic devices.Also, the second semiconductor material preferably has a high absorptioncoefficient suitable for use as an absorber-generator layer in a thinfilm photovoltaic cell. The absorption coefficient is preferably largerthan about 1×10³ /cm and more preferably larger than about 1×10⁴ /cm.These materials, and particularly cadmium telluride and copper indiumdiselenide, are preferred because they can absorb essentially allincident sunlight in a very thin layer. These materials also typicallyhave a very short diffusion length, being less than about five micronsand more preferably less than about one micron. The metal may be asingle metal or a mixed metal, such as when the semiconductor materialcomprises copper indium diselenide. Representative metals includecopper, cadmium, indium gallium, zinc, mercury, lead and combinations ofthe foregoing. Examples of second semiconductor materials includecadmium telluride, copper sulfide, copper indium disulfide, and copperindium diselenide. Preferred metal chalcogenides are Group II-VIsemiconductors (such as cadmium telluride) and Group I-III-VIsemiconductors (such as copper indium diselenide), and particularlythose with a band gap of from about 1 to about 2 eV. Cadmium tellurideand copper indium diselenide are particularly preferred as secondsemiconductor materials.

The first semiconductor film 118 comprises a first semiconductormaterial, which is different than the second semiconductor material.Also, the first semiconductor film has different electrical propertiesthan the first semiconductor material, so as to form the power-producingheterojunction 116 at interfaces between the particles 122 and the firstsemiconductor film 118. In one preferred embodiment, the firstsemiconductor film 118 is n-type, preferably comprising cadmium sulfide,and the second semiconductor film is p-type, preferably comprising atleast one of cadmium telluride and copper indium diselenide, withcadmium telluride being more preferred. In this preferred embodiment,the photovoltaic cell 102 would typically be a backwall design, withsunlight passing through the first electrode film 110 and the firstsemiconductor film 118 prior to being absorbed for power generation inthe second semiconductor film 120. The photovoltaic cell 102 could,however, also be of a frontwall design without deviating from the scopeof the present invention.

The particles 122, and consequently the second semiconductor film 120,are smaller than about 30 microns in size, preferably smaller than about20 microns in size and most preferably smaller than about 10 microns insize. The particles 122 should, however, be large enough to assureessentially complete absorption of available sunlight. The particles 122for preferred second semiconductor materials, such as cadmium tellurideand copper indium diselenide should, therefore, preferably be largerthan about 1.5 microns and more preferably larger than about 2 microns.Particularly preferred are particles 122 are of a size of from about 2microns to about 10 microns. The particles 122 also preferably have arelatively narrow size distribution, such that the particles 122 are ofa relatively uniform size.

The matrix material may be any material which provides for electricalinsulation in the spaces between the particles 122. The matrix materialpreferably has a resistivity of larger than about 15 megaohm-centimeters. Preferred materials for the matrix material 124 includeelectrically insulating polymeric materials such as polyimides,polystyrenes, acrylic resins, and polyurethanes. Sunlight striking thematrix material 124, and not striking the particles 122, represents aloss of power generation capacity. The matrix material 124 may,therefore, include a component or components which could redirect to theparticles 122 at least a portion of the energy of sunlight striking thematrix material 124. In one embodiment, a light reflecting, refracting,or diffusing component could be included in the matrix material 124 toredirect some of the sunlight to the particles 122. Such componentsinclude titanium dioxide, glass, and metals such as aluminum. In anotherembodiment, a fluorescent component, such as fluorescein or a zincsulfide phosphor, could be included in the matrix material 124. Duringoperation of the photovoltaic cell 102, at least some of the fluorescentemission from the fluorescent component would be directed to theparticles 122 to supply additional energy for power production. Theseenergy-redirecting components which could be included in the matrixmaterial 124 may be in the form of very small particles, typicallysmaller than about 0.1 microns. Alternatively, the energy-redirectingcomponent could be present in a homogeneous mixture in the matrixmaterial 124.

The first electrode film 110 and the second electrode film 114 may bemade of any suitable electrode materials which have a high electricalconductivity. The electrode film through which sunlight passes to reachthe semiconductor layer 112 should be optically transmissive, ortransparent to sunlight. This will be the first electrode film 110 whenthe photovoltaic cell 102 has a backwall design. Transparent electrodematerials include transparent conducting oxides such as tin oxide,indium oxide, indium tin oxide, and cadmium stannate. A very thin layerof gold, or another suitable metal such as nickel or chromium, couldalso be used as a transparent electrode film. When gold is used as atransparent electrode film, the film should preferably be thinner thanabout 500 angstroms. The electrode film opposite the sun, which would bethe second electrode film 114 in a backwall cell configuration, does notneed to be transparent and normally comprises a conductive metal, suchas tin, nickel, or chromium. A preferred design for the photovoltaiccell 102 is a backwall design having gold as a first electrode film,n-type cadmium sulfide as a first semiconductor film 118, p-type cadmiumtelluride or p-type copper indium diselenide as the second semiconductorfilm 120, and tin, nickel, or chromium for the second electrode film114. Although the description of the present invention primarily refersto backwall configurations for the photovoltaic cell 102, the sameprinciples apply equally to frontwall configurations making thenecessary adjustments to accommodate the frontwall design.

Although only four film layers are shown in the photovoltaic cell 102 inFIG. 2, additional film layers may be included in the photovoltaic cell102, if desired. For example, FIG. 3 shows one embodiment of thephotovoltaic cell 102, which is the same as shown in FIG. 2 except thatthe photovoltaic cell 102 includes an ohmic contact film 130 interposedbetween the second semiconductor film 120 and the second electrode filmlayer 114. In the case of p-type cadmium telluride for the particles122, an ohmic contact film layer 130 of graphite may be used to improvethe ohmicity of the electrical contact between the second semiconductorfilm 120 and the second electrode film layer 114. Other materials whichmay be used in the ohmic contact layer include tellurium and appropriatecompound semiconductors such as zinc telluride and mercury telluride.These materials may be doped to provide the desired electricalproperties.

In another embodiment, as shown in FIG. 4, the photovoltaic cell 102 mayinclude a protective film 132 to provide a barrier to electrical shortsacross defects which may occur in the first semiconductor film 118 andto improve structural and electrical interconnection of the firstelectrode film 110 and the first semiconductor film 118. The protectivefilm 132 preferably has no significant resistance to current flow acrossthe width of the protective film 132 so as to facilitate free currentflow between the first semiconductor film 118 and the first electrodefilm 110 and a high resistance to current flow laterally across theprotective film. One preferred protective film 132 is a Z-directionconductive tape such as Products Nos. 9703 and 5303R of the 3MCorporation. The Z-direction conductive tape is a tape having anelectrically insulating polymer-based matrix with small conductivemetallic particles dispersed in the matrix and providing a highlyconductive electrical path across the width of the tape. The tape ishowever, substantially electrically non-conductive in a lateraldirection along the tape.

Referring now to FIGS. 5-7, one embodiment of a method for making aphotovoltaic device of the present invention will be described.Referring first to FIG. 5, a flexible, surrogate substrate 140 isprovided having an adhesive on at least one side. The adhesive ispreferably a low tack adhesive which is pressure sensitive. Thesurrogate substrate 140 must be sufficiently flexible and must providesufficient structural support to permit roll to roll processing of thesurrogate substrate during formation of thin films on the surrogatesubstrate 140 for use in a photovoltaic device. The surrogate substrate140 may be made of any flexible materials which could include metals,cloths, plastics, etc. One preferred surrogate substrate 140 is aflexible plastic tape, such as an acetate tape.

In a particle deposition step 142, particles 144 are deposited onto aside of the surrogate substrate 140 having the adhesive. The particles144 may be deposited on the surrogate substrate 140 in any suitablemanner, such as by feeding the particles through a hopper with a feedrate regulated to coincide with the speed at which the surrogatesubstrate is traveling. The particles 144 comprise a crystallinesemiconductor material having a bulk crystal morphology. Thesemiconductor material preferably comprises a compound semiconductor,and more preferably a metal chalcogenide selected from the groupconsisting of a metal sulfide, metal selenide, and metal telluride. Evenmore preferably, the semiconductor material comprises at least one ofcadmium telluride and copper indium diselenide. Also, the particles 144are of a size suitable for use in a thin film photovoltaic cell. Theparticles 144 should be smaller than about 30 microns in size, morepreferably smaller than about 20 microns in size and even morepreferably smaller than about 10 microns in size. Particularly preferredare particles of from about 2 microns in size to about 10 microns insize.

After the particle deposition step 142, the particles 144 are embeddedinto the surrogate substrate 140 in an embedding step 146. The embeddingstep 146 may be accomplished, for example, by pressing the particles 144into the surrogate substrate 140 with a roller. Preferably, greater thanabout 10% of the surface area of the particles 144 are in contact withthe adhesive on the surrogate substrate 140 following the embedding step146. More preferably, greater than about 20% of the surface area of theparticles 144 are in contact with the adhesive.

After the embedding step 146, a film-forming material 150 is applied tothe surrogate substrate 140 to fill spaces which may exist between theparticles 144 in a film-forming material 150 deposition step 152. Thefilm-forming material 150 may be deposited in any suitable manner, suchas by using a doctor blade by spraying, by dipping, or by curtaincoating. The film-forming material 150 deposited on the surrogatesubstrate 140 is preferably deposited to a depth of smaller than about120% of the mean diameter of the particles 144, and more preferably to adepth of from about 90% to about 110% of the mean particle diameter.Preferably, the film-forming material 150 should contain a materialwhich is electrically insulating so that electrical shorts around theparticles 144 may be avoided in a final photovoltaic device. Suitablematerials for the film-forming material include polyamides,polystyrenes, acrylic resins, and polyurethanes. The film-formingmaterial 150 may also comprise one or more components for redirectingenergy of sunlight to the particles 144 in a photovoltaic device, aspreviously described with reference to FIGS. 2-4. Additionally, itshould be noted that as an alternative to separately depositing theparticles 144 and the film-forming material 150, the particles 144 andthe film-forming material 150 could be premixed and deposited as amixture prior to the embedding step 146. In the case of such premixing,the surrogate substrate 140 may be used without an adhesive because itwill not be necessary to adhere the particles 144 to the surrogatesubstrate 144, as previously discussed.

After the film-forming material deposition step 152, the film-formingmaterial 150 is cured in a curing step 154 to impart structural strengthto the film-forming material so that it binds and holds the particles144 in a solid, flexible matrix 155 of the cured film-forming material.During the curing step 154, the film-forming material 150 is generallysubjected to a heat treatment to drive off any volatile solvents and tothermally and/or chemically cross-link the film-forming material toprovide the desired structural properties for the matrix 155. Theheating temperature will depend upon the particular film-formingmaterial used. For example, when using a polyurethane as a film-formingmaterial, a curing temperature of from about 100° C. to about 140° C.should be used.

After the curing step 154, at least a portion of the matrix 155 and theparticles 144 are removed in a first removal step 156. During the firstremoval step 156, a surface portion of the matrix 155 and the particles144 is removed so that substantially planar surfaces 158 of theparticles 144 are exposed. The first removal step 156 may be performedusing any suitable abrading method, such as using a free grit ofabrasion particles or a grit of abrasion particles fixed on a suitablesupport, such as on a rotating wheel, travelling belt, or sliding plate.Preferably, abrasion particles are smaller than about 400 grit.

Referring now to FIG. 6, after the first removal step 156, the exposedsurfaces 158 of the particles 144 are subjected to a first surfacetreatment step 160 in which the surfaces 158 are treated to cleanse thesurfaces 158 of the particles 144 and to prepare the surfaces 158 fordeposition of a film over the surfaces 158. In the first surfacetreatment step 160, oxides and debris which may be present on thesurfaces 158 of the particles 144 are removed. A treating fluid may beused which removes such oxides and debris. Preferred treating fluidsinclude high viscosity alcohols including isobutyl alcohol, propyleneglycol and glycerine, which preferably have a material such as ammoniumbifluoride, potassium cyanide, or hydrogen fluoride in solution. Thetreating fluid may be removed from the surfaces 158 of the particles 144by washing with deionized water, followed by drying. Alternative to, orin addition to, treatment with the relatively mild treating fluid, thesurfaces 158 may be treated with a strong etchant, such as brominedissolved in methanol or a mixture containing hydrofluoric acid andphosphoric acid. The etchant may remove surface irregularities remainingon the surfaces 158 after the first removal step 156. When such anetchant is used, it is preferably followed by treatment with therelatively mild treating fluid.

After the first surface treatment step 160, there is a semiconductorfilm deposition step 162 in which a semiconductor film 164 is depositedover and contacts with the surfaces 158 of the particles 144. Thesemiconductor film 164 comprises a semiconductor material which isdifferent than the semiconductor material of the particles 144, so thatthe semiconductor film preferably forms a heterojunction with thesemiconductor materials of the particles 144. When the semiconductormaterial in the particles 144 comprises cadmium telluride or copperindium diselenide, the second semiconductor material is typicallycadmium sulfide. The semiconductor film 164 may be deposited by anysuitable means such as by chemical vapor deposition, vacuum evaporation(including close spaced sublimation), and solution growth. Preferably,the deposition temperature is lower than about 360° C. to avoid damageto either the surrogate substrate 140 or the bulk crystal morphology ofthe particles 144.

After the semiconductor film deposition step 162, a first electrode film170 is deposited in a first electrode film deposition step 168. Thefirst electrode film 170 should be optically transmissive for a backwallphotovoltaic cell configuration and is preferably a thin metallic layer,such as a thin layer of gold. When using gold, the first electrode film170 should have a film thickness of smaller than about 500 angstroms.The first electrode film 170 may be deposited in the first electrodefilm deposition step 168 by any suitable technique such as by vacuumevaporation.

After the first electrode deposition step 168, the surrogate substrate140 is separated, in a separation step 172, from the other layers, whichform a flexible sheet 174. In some instances, it may be possible toseparate the surrogate substrate 140 from the flexible sheet 174 by theuse of applying a mechanical shock, vibration, or flexure to thesurrogate substrate 140 to cause it to dislodge from the flexible sheet174. Alternatively, a separation blade or other mechanical device couldbe used to peel away the surrogate substrate 140 from the flexible sheet174. The surrogate substrate 140, which has been separated from theflexible sheet 174, may be collected on a separate roll for storageprior to disposal or possible recycling of the surrogate substrate 140,if appropriate. The flexible sheet 174 is further processed.

Referring now to FIG. 7, after the separation step 172, the flexiblesheet 174 is subjected to a second removal step 178, similar to thefirst removal step 146, to remove a portion of the particles 144 fromthe side of the flexible sheet 174 which is opposite the first electrodefilm 170 so that substantially planar, exposed surfaces 180 of theparticles 144 are formed. In one embodiment, however, the second removalstep may be eliminated, if desired. This may be the case when an ohmiccontact layer, such as a graphite layer, is included in the photovoltaicdevice. If the second removal step 178 is eliminated, however, then itis important that the particles 144 project sufficiently out of thematrix 155 to permit physical and electrical contact between theparticles 144 with a subsequently deposited film, such as an ohmiccontact film.

After the second removal step 178, the surfaces 180 of the particles 144are subjected to a second surface treatment step 182, which is similarto the first surface treatment step 160. During the second surfacetreatment, the surfaces 180 are cleansed of oxides and debris to preparethe surfaces 180 for subsequent film deposition. The second surfacetreatment step may comprise treating the surfaces 158 with a treatingfluid and/or an etchant, as previously described.

After the second surface treatment step 182, a second electrode film 188is added to the flexible sheet 174 by deposition over the surfaces 180of the particles 144 in a second electrode deposition step 186. Thesecond electrode film 188 may be made of any suitable electricallyconductive material. For a backwall cell design, the second electrodefilm 188 is typically a conductive metal, such as a tin, chromium,nickel, or other noble metals. The second electrode film 188 may bedeposited by any suitable technique, such as by vacuum evaporation.

After the second electrode deposition step 186, the flexible sheet 174is cut into separate pieces in the form of flexible strips 190 in acutting step 189. Each flexible strip 190 comprises a photovoltaic cellsubstantially as described with respect to FIG. 2. It should berecognized, however, that it is not necessary for all of the film layersof a photovoltaic cell to be a part of the flexible strip 190. One ormore film layers could be omitted from the flexible strip 190 and theomitted film layer or layers could be separately provided and combinedwith the flexible strip 190 to form a complete photovoltaic cell.Furthermore, the flexible strip 190 could be made to comprise additionalor different film layers than those which have been described withreference to FIGS. 5-7, so long as the flexible strip 190 includes atleast the particles 144 in the matrix 155. For example, an ohmic contactfilm could be included between the film having the particles 144 in thematrix 155 and the second electrode film 188. This ohmic contact layercould comprise graphite deposited by roller coating directly over thesurfaces 180 of the particles 144 prior to deposition of the secondelectrode film 188. The graphite could contain a dopant, such as copper,which could be diffused into the particles 144 by heat treatment toimprove the ohmicity of the electrical contact between the particles 144and the second electrode film 188. Materials other than graphite may beused for the ohmic contact layer, as previously disclosed. Having suchan ohmic contact film is particularly useful when the particles 144comprise cadmium telluride as a semiconductor material.

Also, deposition of the semiconductor film 164 may be avoided by usingparticles 144, which when deposited, have a first semiconductor materialin the interior of the particles 144 and a thin coating of a secondsemiconductor material at an exposed surface of the particles 144, withthe two semiconductor materials forming a heterojunction. Such particlescould be made, for example, by exposing cadmium telluride particles to agaseous hydrogen sulfide environment to convert a portion of the cadmiumtelluride in the surface region to cadmium sulfide. In such a case, thesecond removal step 178 would not be used and a second electrode filmcould be deposited directly on the surfaces 158 after the first surfacetreatment step 160, with or without an ohmic contact interlayer. Afterremoval of the sacrificial substrate in the separation step 172, a firstelectrode film could then be deposited over exposed surfaces ofparticles 144 to complete a photovoltaic cell.

Another alternative to depositing the semiconductor film 164 would be toinstead convert a first semiconductor material in the particles 144adjacent to the surfaces 158, following the first surface treatment step160, to a second semiconductor material. For example, when the particles144 comprise cadmium telluride, the exposed surfaces 158 of theparticles 144 could be subjected to gaseous hydrogen sulfide to converta surface portion of the cadmium telluride to cadmium sulfide. Thecadmium telluride and cadmium sulfide would then form the desiredheterojunction. In such a surface treatment, however, it should berecognized that the bulk crystal morphology of the cadmium telluride inthe particles 144 should be substantially unaffected. As used herein,bulk crystal morphology refers to the crystal morphology of thecrystalline semiconductor material in the interior portion of theparticles 144. For absorber-generator semiconductor materials such ascadmium telluride, therefore, the bulk crystal morphology is themorphology of the crystalline cadmium telluride which will beresponsible for absorbing sunlight for power generation during operationof a photovoltaic device. It should be recognized that alterations ofcrystal morphology in the surface regions of the particles 144 do notaffect the bulk crystal morphology, which is substantially retainedthroughout the process of the present invention. No recrystallization ofthe semiconductor material in the particles 144 occurs according to thepresent invention. Defect chemistry changes may, however, occur. Forexample, cadmium vacancies in a cadmium telluride film could move orreadjust due to processing, treatment, or relaxation over time.

The particles 144, of the desired size, may be obtained by comminutinglarge crystalline chunks which are commercially available in bulk.Comminution may involve, for example dry or wet grinding or milling.Preferably, a narrow size distribution of the particles 144 is obtainedby sieving or otherwise classifying the particles 144. Preferably,substantially all of the particles 144 are smaller than about 50% largerthan the mean particle diameter of the particles 144. More preferably,substantially all of the particles 144 are from about 50% smaller toabout 50% larger than the mean particle diameter.

In one embodiment, once the particles 144 having the desired size havebeen obtained, the particles 144 are subjected to a first pre-treatmentprior to incorporation into the flexible sheet 174. During thepre-treatment, the properties of the semiconductor material in theparticles 144 are levelized, so that all of the particles 144 havesubstantially the same properties throughout each particle and the oxidelevel of the particles is significantly reduced or adjusted. Such firstpre-treatment typically involves heating the particles in a reducingatmosphere, such as in an atmosphere of hydrogen gas, at a temperatureabove about 500° C., to drive off oxygen which may be present as anoxide on the particles 144 and to levelize the composition of theparticles 144 such as by driving off any excess of residual material.Such a first pre-treatment is particularly preferred when using cadmiumtelluride as the semiconductor material in the particles 144.

A second pre-treatment of the particles 144 may also be performed priorto incorporating the particles 144 into the flexible sheet 174. Thissecond pre-treatment may be performed independent of or in addition tothe first pre-treatment. When used with the first pre-treatment, thefirst pre-treatment should precede the second pre-treatment. The secondpre-treatment comprises subjecting the particles 144 to oxygen gas in anoxidizing environment at a temperature of from about 300° C. to about500° C. for a short period of time in order to oxidize impurities whichmay be present in the particles to passivate the impurities.

Following the manufacture of the flexible strips 190, a plurality of theflexible strips 190 may be located adjacent to a supporting substrateand used to form a photovoltaic module having a plurality ofphotovoltaic cells commonly supported by the supporting substrate. Thephotovoltaic cells may be electrically interconnected, preferably inseries, on the common supporting substrate.

Referring to FIGS. 8-10, one embodiment is shown for locating aplurality of flexible strips 190, having a photovoltaic cellconfiguration as shown in FIG. 2, on a supporting substrate 200, such asa sheet of glass, to form a photovoltaic module having a plurality ofphotovoltaic cells. Referring first to FIG. 8, a first bead 202a ofelectrically conductive paste is placed across the supporting substrate200 and a first flexible strip 190a is positioned to overlap the firstbead 202a of electrically conductive paste. The first flexible strip190a comprises a first electrode film 110a, a first semiconductor film118a, a second semiconductor film 120a, and a second electrode film114a. The first flexible strip 190a is positioned with the firstelectrode film 110a facing the supporting substrate 200. Theelectrically conductive paste of the first bead 202a may comprise anysuitable electrically conductive material such as in a viscous slurryform, such as a copper or nickel paste or a copper epoxy. The first bead202a may be deposited on the supporting substrate 200 by any suitablemeans, such as by deposition in a line across the supporting substrate200 from a small diameter tube or needle or by offset printing.

Referring now to FIG. 9, the first flexible strip 190a is pressed intothe first bead 202a to form an electrical contact between the first bead202a and the first electrode film 110a of the first flexible strip 190a.The first bead 202a may then be cured to remove any volatile materialand to structurally set the conductive material of the first bead 202a.Such curing typically involves heating of the bead 202a to an elevatedtemperature. Alternatively, the curing may be postponed until after aplurality of flexible strips 190 have been located on the supportingsubstrate 200.

Also as seen in FIG. 9, a first strip 204a of electrically insulatingmaterial is deposited over an edge of the first flexible strip 190aopposite from the first bead 202a. Such a first strip 204a couldcomprise any composition having a suitable electrically insulatingmaterial. One useful composition for preparing the first strip 204a is aslurry of titanium dioxide with a binder, such as ethyl cellulose, in acarrier liquid, such as hexanol. The first strip 204a could then becured in a subsequent heating step, either independent of orsimultaneous with curing of the first bead 202a. On top of the firststrip 204a is deposited a second bead 202b of the electricallyconductive paste and a second flexible strip 190b is located to overlapthe second bead 102b.

Referring now to FIG. 10, the second flexible strip 190b is shownpressed into the second bead 202b such that the second electrode film114a of the first flexible strip 190a, is in series electricalinterconnection with the first electrode film 110b of the secondflexible strip 190b. Also shown in FIG. 10 is a second strip 204b of theelectrically insulating material and a third bead 202c of theelectrically conductive material prepared to accept another flexiblestrip 190 for electrical interconnection. Any number of flexible strips190 may thus be located and electrically interconnected in series on thecommon supporting substrate 200 to provide a photovoltaic module havinga desired number of electrically interconnected photovoltaic cells. Thefirst and last photovoltaic cells in series could be connected toelectrical busses to facilitate removal of current from the photovoltaicmodule during operation and to distribute current into the photovoltaicmodule during operation.

The plurality of interconnected photovoltaic cells supported on the basesubstrate 200 may be encapsulated on the supporting substrate 200 toprotect the photovoltaic cells from the external environment. Anysuitable encapsulation technique may be used such as that disclosed inco-pending U.S. patent application Ser. No. 08/095,381, now U.S. Pat.No. 5,460,660.

As noted previously with reference to FIGS. 5-7, the flexible strips 190need not have all films necessary to make a photovoltaic cell. FIGS.11-13 show one embodiment of the present invention for forming aplurality of interconnected photovoltaic cells on the supportingsubstrate 200 using flexible strips of 190 which do not comprise all ofthe films of a complete photovoltaic cell. Referring first to FIG. 11,the base substrate 200 is provided having a plurality of first electrodefilms 110a, 110b supported on the base substrate 200. The first flexiblestrip 190a comprises a first semiconductor film 118a, a secondsemiconductor film 120a, and a second electrode film 114a, all of a typeas previously described with reference to FIGS. 2-4. The first electrodefilms 110a, 110b are separated by a photovoltaic cell interconnectionregion 212. On top of the first electrode film 110a is placed a firstZ-direction conductive film 132a. Positioned above the first Z-directionconductive film 132a is the first flexible strip 190a. The firstZ-direction conductive film 132a may be a Z-direction conductive tapesuch as previously described with reference to FIG. 4. Such Z-directionconductive tape has an adhesive on both surfaces so that it may beadhered to the first electrode film 110a on one surface by theapplication of pressure and may be then adhered on the other surface tothe first semiconductor film 118a of the first flexible strip 190a, alsoby application of pressure.

Referring to FIG. 12, the first flexible strip 190a is adhered to thefirst Z-direction conductive film 132a to form a complete firstphotovoltaic cell comprising the film layers of the flexible strip 190a,the first Z-direction conductive film 132a, and the first electrode film110a. The photovoltaic cell would, therefore, have a configuration aspreviously described with reference to FIG. 4. For a backwallphotovoltaic cell configuration, the first electrode film 110a and thefirst Z-direction conductive film 132a should be optically transmissive.The supporting substrate 200 should also be optically transmissive, suchas a sheet of glass, for the embodiment shown in FIGS. 11-13. The firstelectrode film 110a may be, for example, a tin oxide film and the firstsemiconductor film 118a may be cadmium sulfide.

Also shown in FIG. 12, the first strip 204a of electrically insulatingmaterial is placed at an edge of the first flexible strip 190a. Anelectrically conductive interconnection film 216a is deposited over thefirst strip 204A to connect the second electrode film 114a of the firstflexible strip 190a with the first electrode film 110b. On the firstelectrode film 110b is placed a second z-direction conductive film 132b,above which is positioned a second flexible strip 190b.

Referring to FIG. 13, a second photovoltaic cell is shown completedwhich comprises the first electrode film 110b, the second Z-directionconductive film 132b, and the film layers of the second flexible strip190b. The second photovoltaic cell is electrically interconnected withthe first photovoltaic cell through the electrically conductiveinterconnection film 216a. A second strip 204b of electricallyinsulating material has been placed at the edge of the secondphotovoltaic cell in anticipation of electrical interconnection with oneor more additional photovoltaic cells which could be added in aphotovoltaic module.

It should be recognized that, according to the embodiment as describedwith respect to FIGS. 11-13, it would be possible to first locate all ofa plurality of photovoltaic cells on the supporting substrate 200 andthen to electrically interconnect all photovoltaic cells by screenprinting or offset printing, in a single application, a plurality ofstrips 204 of electrically insulating material, followed by screenprinting or offset printing of a plurality of electrically conductiveinterconnecting strips 216.

As described with reference to FIGS. 5-13, the present inventionprovides a method in which particles of a semiconductor material may beincorporated into a photovoltaic module substantially without alteringthe bulk crystal morphology of the semiconductor material in theparticles. This represents a substantial improvement over existingmethods, such as those used to prepare cadmium telluride-containingphotovoltaic devices, in which recrystallization of a semiconductormaterial occurs following deposition in order to obtain suitable crystalmorphology for use in the photovoltaic module.

It should be recognized that any of the features of any embodimentdescribed herein may be combined in any combination with any otherembodiment described herein. For example, any of the features of thephotovoltaic cells of FIGS. 2-4 may be combined with any of the otherfeatures of any of the photovoltaic cells of FIGS. 2-4. Any combinationof films of any of the photovoltaic cells of FIGS. 2-4, which includethe second semiconductor film 120, may be prepared according to theprocess described with reference to FIGS. 5-7. Any flexible stripsprepared according to the method described with respect to FIGS. 5-7 maybe used with any of the embodiments described with reference to FIGS.8-13 for making a photovoltaic module having a plurality of electricallyinterconnected photovoltaic cells supported on a common base substrate.

Also, while various embodiments of the present invention have beendescribed in detail, it is apparent that modifications and adaptationsof those embodiments will occur to those skilled in the art. It is to beexpressly understood, however, that such modifications or adaptationsare within the scope of the present invention, as set forth in thefollowing claims.

What is claimed is:
 1. A method for making a photovoltaic device whichreduces the possible need for the bulk recrystallization of asemiconductor material during the manufacture of the photovoltaicdevice, the method comprising the steps of:providing particlescomprising a first semiconductor material in crystalline form having abulk crystal morphology, said semiconductor material being one of n-typeand p-type; making, after said step of providing, a semiconductor layercapable of producing electrical energy in response to solar energystriking said semiconductor layer; said semiconductor layer including afirst semiconductor film, of a thickness that is smaller than about 30microns, having said first semiconductor material of said particles;said step of making said semiconductor layer comprising providing asurrogate substrate, forming said first semiconductor film adjacent toand supported by said surrogate substrate, and separating said film fromsaid surrogate substrate such that said first semiconductor film is nolonger supported by said surrogate substrate; said semiconductor layerincluding a second semiconductor film, having a second semiconductormaterial, located adjacent to said first semiconductor film to form aheterojunction between said first semiconductor material and said secondsemiconductor material; and electrically interconnecting saidsemiconductor layer with a first electrode and a second electrode;wherein, said particles are smaller than about 30 microns in size andsaid bulk crystal morphology of said first semiconductor material issubstantially retained between said step of providing said particles andsaid step of electrically interconnecting.
 2. The method of claim 1,wherein:said first semiconductor film has a thickness of smaller thanabout 20 microns.
 3. The method of claim 1, wherein:said firstsemiconductor film comprises said particles and spaces between saidparticles, said spaces between said particles being at least partiallyoccupied by a matrix material which is different than said semiconductormaterial.
 4. The method of claim 3, wherein:said matrix material iselectrically insulating.
 5. The method of claim 3, wherein:said matrixmaterial comprises a polymeric material.
 6. The method of claim 1,wherein:said step of making said semiconductor layer further comprisesremoving a portion of said first semiconductor film from at least onesurface of said first semiconductor film such that relatively planarsurfaces of said particles of first semiconductor material are exposedat a surface of said first semiconductor film.
 7. The method of claim 1,wherein:said first semiconductor film, after said step of separating, islocated adjacent to and supported by a permanent supporting substratewhich is different than said surrogate substrate, said film forming apart of a photovoltaic cell supported by said supporting substrate. 8.The method of claim 1, wherein:an electrode film is located adjacent tosaid second semiconductor film on a side of said second semiconductorfilm opposing said first semiconductor film, said electrode film beingelectrically interconnected with said second semiconductor film.
 9. Themethod of claim 8, wherein:said step of locating said electrode filmadjacent to said second semiconductor film occurs prior to said step ofseparating.
 10. The method of claim 8, wherein:said step of locatingsaid electrode film adjacent to said second semiconductor film occursafter said step of separating; and an interlayer is disposed betweensaid second semiconductor film and said electrode film; wherein, saidinterlayer is substantially electrically conductive in a directionacross the thickness of said interlayer between said secondsemiconductor film and said electrode film, and is substantiallyelectrically non-conductive in a direction laterally across saidinterlayer.
 11. The method of claim 1, wherein:said particles aresmaller than about 20 microns in size.
 12. The method of claim 1,wherein:said particles are smaller than about 10 microns in size. 13.The method of claim 1, wherein:said particles are from about 2 micronsto about 10 microns in size.
 14. The method of claim 1, wherein:saidfirst semiconductor material comprises a compound semiconductor.
 15. Themethod of claim 1, wherein:said first semiconductor material comprises ametal chalcogenide selected from the group consisting of a metalsulfide, a metal selenide, and a metal telluride.
 16. The method ofclaim 15, wherein:said metal chalcogenide comprises a metal selectedfrom the group consisting of copper, cadmium, zinc, indium, gallium,mercury, lead, and combinations thereof.
 17. The method of claim 15,wherein:said first semiconductor material comprises a first metalchalcogenide and said second semiconductor material comprises a secondmetal chalcogenide different from said first metal chalcogenide.
 18. Themethod of claim 17, wherein:said first semiconductor material comprisesat least one of cadmium telluride and copper indium diselenide.
 19. Themethod of claim 17, whereinsaid second metal chalcogenide is cadmiumsulfide.
 20. The method of claim 1, wherein:said first semiconductormaterial has an absorption coefficient of greater than about 1×10³. 21.A photovoltaic device, the photovoltaic device comprising:asemiconductor layer capable of producing electrical energy in responseto solar energy striking said semiconductor layer; and a first electrodeand a second electrode, each of which is electrically interconnectedwith said semiconductor layer; said semiconductor layer comprising afirst semiconductor film of a thickness of smaller than about 30microns, said first semiconductor film including particles comprising afirst semiconductor material in crystalline form and having spacesbetween said particles, wherein said spaces between said particles areat least partially occupied by an electrically insulating composition toreduce the potential for electrical shorting through said film; saidsemiconductor layer also comprising a second semiconductor film,adjacent said first semiconductor film, comprising a secondsemiconductor material, wherein said first semiconductor film and saidsecond semiconductor film form a heterojunction for photovoltaic powerproduction; an interlayer being located between said secondsemiconductor film and said second electrode, said interlayer beingsubstantially electrically conductive in a direction across thethickness of said interlayer and being substantially electricallynon-conductive in a direction laterally across said interlayer.
 22. Thephotovoltaic device of claim 21, wherein:said electrically insulatingcomposition has a bulk resistivity of larger than about 15 megaohm-centimeters.
 23. The photovoltaic device of claim 21, wherein:saidinsulating composition comprises a polymeric material.
 24. Thephotovoltaic device of claim 21, wherein:said particles are smaller thanabout 20 microns in size.
 25. The photovoltaic device of claim 21,wherein:said particles are smaller than about 10 microns in size. 26.The photovoltaic device of claim 21, wherein:said particles are of asize from about 2 microns to about 10 microns.
 27. The photovoltaicdevice of claim 21, wherein:said first semiconductor material comprisesa compound semiconductor.
 28. The photovoltaic device of claim 21,wherein:said first semiconductor material comprises a metal chalcogenideselected from the group consisting of a metal sulfide, a metal selenide,and a metal telluride.
 29. The photovoltaic device of claim 28,wherein:said metal chalcogenide comprises at least one of cadmiumtelluride and copper indium diselenide.
 30. The photovoltaic device ofclaim 21, wherein:said first semiconductor material has an absorptioncoefficient of larger than about 1×10³.
 31. The photovoltaic device ofclaim 21, wherein:the photovoltaic device further comprises, in saidspaces between said particles, means for redirecting energy toward saidparticles from sunlight not directly striking said particles.
 32. Thephotovoltaic device of claim 21, wherein:said electrically insultingcomposition, located in said spaces between said particles, comprisesmeans for reflecting, diffusing, or refracting sunlight striking saidelectrically insulating material to redirect at least a portion ofenergy in said sunlight to at least one of said particles.
 33. Thephotovoltaic device of claim 21, wherein:the photovoltaic device furthercomprises, in said spaces between said particles, a fluorescent materialwhich is capable of producing a fluorescent emission in response tosunlight such that at least a portion of said fluorescent emission maybe absorbed by said particles.
 34. The photovoltaic device of claim 21,wherein:said spaces are substantially entirely occupied by saidelectrically insulating composition.
 35. The photovoltaic device ofclaim 21, wherein said first semiconductor material comprises a firstmetal chalcogenide and said second semiconductor material comprises asecond metal chalcogenide that is different from said first metalchalcogenide.
 36. The photovoltaic device of claim 35, whereinsaidsecond semiconductor material comprises cadmium sulfide.
 37. Thephotovoltaic device of claim 21, wherein:said direction laterally acrosssaid interlayer is substantially perpendicular to said direction acrosssaid thickness of said interlayer.
 38. A method for making aphotovoltaic device which reduces the possible need for bulkrecrystallization of a semiconductor material during the manufacture ofthe photovoltaic device, the method comprising the steps of:providingparticles comprising a semiconductor material in crystalline form havinga bulk crystal morphology; making, after said step of providing, asemiconductor layer including said semiconductor material of saidparticles, said semiconductor layer being capable of producingelectrical energy in response to solar energy striking saidsemiconductor layer; and electrically interconnecting said semiconductorlayer with a first electrode and a second electrode; wherein, saidparticles are smaller than about 30 microns in size and said bulkcrystal morphology of said semiconductor material is substantiallyretained between said step of providing said particles and said step ofelectrically interconnecting; and wherein, disposed between said secondelectrode and said semiconductor layer is an interlayer, said interlayerbeing substantially electrically conductive in a direction across thethickness of said interlayer and being substantially electricallynon-conductive in a direction laterally across said interlayer.